1. Field of the Invention
The method of this invention relates to hydraulic fracturing of a subterranean formation by a fracturing fluid. Particularly, this invention relates to the design of techniques and selection of materials for fracturing a formation. More particularly, this invention relates to a method for interpreting bottomhole fracturing pressure of a formation and the use of such interpretations for alteration and design of such techniques.
2. Setting of the Invention
Oil and gas accumulations usually occur in porous and permeable underground rock formations. In order to produce the oil and gas contained in a formation, a well is drilled into the formation. The oil and gas may be contained in the porosity or pore spaces of the formation, hydraulically connected by means of permeability or interconnecting channels between the pore spaces. After the well is drilled into the formation, oil and gas are displaced to the wellbore by means of fluid expansion, natural or artificial fluid displacement, gravity drainage, capillary expulsion, etc. These various processes may work together or independently to move the hydrocarbons into the wellbore through existing flow channels. In many instances, however, production of the well may be impaired by drilling fluids that enter into and plug 4 the flow channel 5, or is unsatisfactory due to insufficient natural channels leading into the particular bore hole or insufficient permeability surrounding the borehole which may result in a noncommercial well. The problem then becomes one of treating the formation in a manner which will increase the ability of a formation rock to conduct fluid to the wellbore.
Various methods of hydraulically fracturing a formation to increase the conductivity of the formation have been developed. Hydraulic fracturing may be defined as a process in which fluid pressure is applied to exposed formation rock until total failure or fracturing occurs. After failure of the formation rock, a sustained application of fluid pressure extends the crevice or fracture outward from the point of failure. The fracture, held open by a proppant, creates a high capacity flow channel and exposes new surface area along the fracture. Although dependent upon overburden pressure, fractures below about 3000 feet are generally vertical. The method of this invention is most useful, is not solely, in generation of vertical fractures.
It is desirable in forming such vertical fractures that the height of such a vertical fracture should be confined to approximately the zone of interest to maximize the length of the fracture formed with the fluid injected or time expended. One method of controlling height during vertical fracturing includes maintaining a low pumping rate. Use of low rates alone may not necessarily alleviate undesirable fracturing which can occur at high bottomhole treating pressures and does require longer than normal fracturing times.